This invention pertains to an article (e.g., a multimode fiber local area network, or means for coupling light into a multimode fiber) that comprises means for mode-selectively launching signal radiation from a semiconductor light source into a multimode fiber (MMF). It also pertains to a method for a mode-selective launch.
A MMF is an optical waveguide that supports more than one (typically dozens or even hundreds) guided mode. In an ideal MMF for data transmission, the various modes all have equal group velocities. In practice, however, the common fiber manufacturing processes (e.g., MCVD, OVD) frequently leave perturbations in the refractive index profile of the fiber, typically at or near the center of the core of the fiber. These perturbations tend to adversely affect the group velocities of the low order modes, resulting in decreased bandwidth of the fiber. More specifically, known MMF typically experiences very small coupling of optical power between the various modes. Hence, a pulse of light launched into the lower order modes as well as the high order modes of the MMF will frequently split into two or more separate pulses, which is clearly undesirable for data transmission.
German patent DE 19809823 C1 discloses means for coupling light from a transmitter 1 into a multimode optical fiber 18. The means comprise an adapter fiber 4 (preferably a single mode fiber; see column 2, lines 60-61), which is coupled to the multimode fiber 18 in a plug-type coupler. The input portion of the multimode fiber is positioned axially in ferrule 15, and the output portion of the adapter fiber is positioned in ferrule 8 such that the output portion makes an acute angle xcex1(2-15xc2x0) with the axis of ferrule 8. The patent asserts (see column 3, lines 50-57) that the tilted disposition of the adapter fiber in the ferrule results in increased filling of the core of the MMI with guided modes, allegedly resulting in increased bandwidth of the multimode fiber. In any case, the coupling means of the German patent have the disadvantage of relatively high cost, due to the need for the off-axis bore in ferrule 8.
It is known that launching pulses simultaneously into both the low and high order modes should be avoided. See, for instance, L. Raddatz et al., IEEE Photonics Technology Letters, Vol. 10, pages 534-536, (1998). The offset launch method proposed by Raddatz et al. to avoid launching optical power into low-order modes was written into the Gigabit Ethernet standard. In this method, a narrow beam of light is launched perpendicular to the end-face of a MMF, offset a distance from the center of the fiber. Substantially no power is launched into the lower-order modes because the beam does not overlap any of these modes.
Although the above mentioned off-center launch can provide acceptable bandwidth, in practice it has been found to be at best inconvenient to provide the off-center launch. Indeed, manufacturability would be enhanced if the beam could be directed at the center of the end-face of the MMF. This application discloses a technique for mode-selective center launch, and apparatus for such launch.
All herein cited references are incorporated herein by reference.
Glossary and Definitions
By xe2x80x9claunch-related membersxe2x80x9d we mean herein the components of a multi-mode optical fiber system that include the output surface of the signal radiation source and the input face of the multimode fiber, and further includes the optional optical components that are disposed in the light path from the signal radiation source to the input face of the multimode fiber. Associated with each launch-related member is at least one surface that is being traversed by the signal radiation beam from the signal radiation source. The launch-related members typically are disposed such that the direction normal to the output surface of the signal radiation source is not parallel to the direction normal to the input face of the multimode fiber but makes an angle xcex8b therewith.
Herein the modes of a multimode optical fiber are designated in conventional fashion LPij, where i=0, 1, 2, . . . . and j=1, 2, . . . LP01 is the fundamental mode. All modes that share a common value of 2j+i also share a common wave number, and are said to constitute a xe2x80x9cprincipal mode groupxe2x80x9d. The quantity 2j+ixe2x88x921 is called the index of the group. A xe2x80x9clower order modexe2x80x9d herein is a mode belonging to a principal mode group with small index, e.g., 1, 2 or 3.
A light beam is xe2x80x9cnominallyxe2x80x9d directed at the center of a multimode optical fiber if any departure from center incidence is unavoidably due to manufacturing limitations. Such departure will typically be less than about 4 xcexcm. Launch of optical power a given lower order mode is xe2x80x9csubstantiallyxe2x80x9d avoided if the power in that mode is at most 10% of the power in that mode for xcex8b=0.
The xe2x80x9caxisxe2x80x9d of a semiconductor light source is, in conventional fashion, the direction normal to the output face of the light source in the center of the light beam.
The xe2x80x9caxisxe2x80x9d of the multimode fiber is, in conventional fashion, the axis of symmetry of a circularly symmetric fiber.